Lona ran a network that included engineers and technicians at munitions and aviation plants in the New York area after Morris had been drafted in 1942. A source of heres smuggled a working model of a new machine gun out of a munition plant. The FBI discovered later that Lona had worked at two defense plans, the Public Metal Company in New York City in 1941 and the Aircraft Screw Products plant on Long Island in 1943.

She was a courier who picked up reports from Klaus Fuchs (covernames CHARLES and REST), David Greenglass (covernames BUMBLEBEE and CALIBRE), Theodore Hall (covername YOUNGSTER [MLAD]), and a source covernamed "FOGEL" and "PERS" from the American secret atomic weapons project at Los Alamos, New Mexico and carried them to the Soviet consulate in New York, where a KGB sub-residency under a young engineer named Leonid R. Kvasnikov (covername ANTON) coordinated operations and dispatched intelligence to Moscow.

After the defections of Elizabeth Bentley and Igor Gouzenko, the Cohens ended contact with Soviet intelligence until 1949, at which time they began working with Col. Rudolph Abel, the Illegal U.S. Rezident. After Fuchs had been arrested in Great Britain in 1950, the Cohens fled to Moscow. Here Lona Cohen received additional training for the work as radio operator and cipher clerk. In 1954 the pair resurfaced in London under the names Helen and Peter Kroger with new Zealand passports. They set up a antiquarian book dealer business which was cover for their activities of running the London Illegal Rezidentura, headed by Morris. Gordon Lonsdale worked with them.

In the basement of their house, situated not far from the military airfield Of Nortkholt, they set up a radio station and began transferring to Moscow "information of special importance" with the use of the high speed radio transmitter. In January of 1961 they were arrested for espionage and in March Lona received a sentence of 20 years and Morris 25 years.

In 1969 the two were exchanged with the Soviet Union for a British subject being held. Back in Moscow, they two continued training colleagues for illegal intelligence operations.

Lona Cohen was awarded the Order of the Red Banner and Order of Order of Friendship of Nations. Her code name in Soviet intelligence and the Venona files is "Lesley".